Rotary vane pumps are well known as means of creating vacuums. Rotary vane pumps are positive-displacement pumps that function by vanes mounted to a rotor rotating inside of a cavity defined by a stator.
In a preferred embodiment, the rotor is of a smaller radius than the radius of the stator and is eccentrically mounted such that the axis of the rotor is displaced from the axis of the stator leaving the rotor and vanes to turn freely within the stator.
The vanes can be variable length and may be tensioned to maintain contact with the walls as the rotor rotates. If the vanes are not tensioned, centrifugal force developed while the rotor turns will drive the vanes outward maintaining contact with the stator.
Vanes may be made of a durable natural or synthetic material. Kevlar (trademark) is used in a preferred embodiment. The choice of material allows the vanes to be worn down while maintaining a seal with the stator.
Lubricants can be used in the system to ensure a seal between the vanes and the stator. If lubricants are used in vacuum applications, provision must be made to ensure their removal before the gases or fluids being pumped are exhausted from the system. Such filter systems are well known in the art.
Vanes may be mounted radiating from the axis of the rotor. The vanes may also be angled into the direction of the rotor's rotation to create a scoop effect.
Rotary vane pumps have been known since at least 1874. CA3559 issued to Barnes describes a hand-operated rotor with vanes which are said to slide diametrically from rotation. The invention also shows inlet and outlet ports.
When operated in order to generate a vacuum, a rotary vane pump has a number of practical operating parameters. In industrial applications, the vacuum pressure (stated as inches or centimetres of mercury or pounds per square inch) and the amount of air flow (stated as volume per time such as cubic feet per minute) are used as an indication of the pump's capacity.
In certain industrial applications, the size and weight of a rotary vane vacuum pump are important considerations. For example, a rotary vane vacuum pump that will be used in a mobile environment must be sufficiently large for the pumping task at hand yet sufficiently light that the fuel requirement of the ongoing transportation of the pump is reduced.
It is known that in designing rotary vane vacuum pumps an important design consideration is the size and location of the inlet port in relation to the outlet port. (See: Ramprasad and Radha, On some design aspects of rotary vane pumps. Vacuum 23:7 page 245) However, until the present invention, no practical proposals has been made how to achieve a suitably sized and disposed inlet port.
Some rotary vacuum pumps have provided for multiple inlet ports. U.S. Pat. No. 2,314,056 to Sobek shows multiple small sized inlet ports disposed along a longer inlet channel thereby achieving a cooling function as the cooler inlet air is circulated around the stator. However, the long distance the air needs to travel likely reduces the increased function that the multiple air inlet ports might have.
Another possibility to increase the amount of inlet air is to increase the size of the inlet port and to provide means that the inlet air will be exposed to the vanes for a longer distance of travel of the vanes. Application DE19853104 to Song suggests the extension of the inlet port in this fashion without actually providing means on how to accomplish this function in practice.
Another possibility to increase the amount of inlet air is to create additional ports. U.S. Pat. No. 7,207,782 to Heaps provides for an additional inlet port. This in turn also requires non return valves and involves more complex inlet geometry and manufacturing to create the additional inlet port in the stator.
Although it is known that the efficiency and throughput of a vacuum pump can be increased by increasing the size of the inlet ports in the body of the stator, a number of different practical problems arise.
One of the practical problems in increasing the size of the inlet ports in the body of the stator is their possible mechanical interaction with the vanes. For example, a port that is elongated in the parallel direction of a vane risks the possibility that as a vane rotates past the port, the vane may become jammed in the port. In order to prevent this from happening, at any given position, the port opening in the stator must represent only a small portion of the width of the vane.
Another practical problem in designing inlet ports is the amount of arc that a vane will travel perpendicular to the rotation of the rotor across the inlet opening. The actual compression of air required in the pump will not begin until the vane passes the last open portion of the opening. Therefore, the total arc length of the port must be considered to ensure that the efficiency of the pump is not adversely affected.
The number of vanes in the rotor can also affect the efficiency of a pump. The minimum number of vanes is two disposed on alternate sides of the rotor. More vanes can be used spaced equally around the rotor. More vanes means a smaller volume of gas will be enclosed and compressed in any individual space formed between the vanes.
The number of vanes will also affect the sizing of the inlet port. The inlet port will normally be sized taking into account how many different voids between vanes will be covered by the inlet port recognizing that compression in any specific void will not take place until that void has passed by the inlet port opening.
Another challenge in designing inlet ports in the stator is to ensure that appropriate mechanical support is provided at appropriate points to maintain the overall structural integrity of the stator. The stator endures many mechanical and thermal stresses. Appropriate supports to maintain the integrity of the stator particularly around the inlet port is appropriate.